Formed product and method for production thereof

ABSTRACT

A formed product, characterized in that it is produced by using, as a material, a steel having an ultra fine structure comprising ferrite grains having an average grain diameter of 3 μm or less and by a method comprising only a forming step and including none of refining steps; and a method for producting the above formed product with ease. The formed product contains no alloying element and has been subjected to no refining step, and has been imparted with high strength and high toughness by the above ultra fine structure.

TECHNICAL FIELD

The present invention relates to a formed product, and a method forproduction thereof. More particularly, the invention relates to a formedproduct ensuring high strength and high toughness by ultra finestructure without resort to addition of alloying elements or refiningsteps, and a method of producting the same easily.

BACKGROUND ART

Hitherto, to product a formed product of metal or alloy, generally, asteel bar or wire is used as material, and formed in cold or warmworking, and refined by hardening, carburizing or tempering. Inproduction of formed product, however, refining by hardening andtempering is a complicated and difficult process, and if producedwithout refining steps, the productivity is enhanced and it is verybeneficial industrially.

Herein, formed products include screws, bolts, nuts, shafts, rivets,pins, stud bolts, fasteners, gears, shaft parts, springs, and othermachine parts (Structural steels for machines, by Toshiyuki Watanabe,published by Japan Iron and Steel Society, p. 46, p. 97).

Recently, in the field of screws and bolts, it is possible to productwithout refining steps in JIS strength classification of up to 8.8. Inthe case of production without refining, it is required to enhancestrength of the material itself, and alloying elements such as Cr, Ti,Nb, or B are added to the material. However, addition of alloyingelements often lowered the toughness of formed product, and was notalways preferred means. In JIS strength classification of up to 8.8,however, most screws and bolts are now produced by conventional methodincluding refining steps. As the production method for high strengthscrews and bolts having a tensile strength of 800 MPa or more, therefining process by hardening and tempering is indispensable.

The invention is devised in the light of the above background, and it ishence a primary object thereof to solve the problems of the prior art,and present a formed product of high strength holding the strength byultra fine structure without resort to addition of alloying elements orrefining steps, for example, pressed product, various parts and members,and a method for production capable of producting such formed product ofhigh strength, such as screws and bolts, easily.

DISCLOSURE OF THE INVENTION

To solve the above problems, the invention presents the following.

It is a first aspect of the invention to present a formed productcharacterized by having an ultra fine structure comprising ferritegrains of average grain diameter of 3 μm or less, and it is a secondaspect to present a formed product characterized by using a steel havingan ultra fine structure comprising ferrite grains of average graindiameter of 3 μm or less as raw material, and being produced by aforming step only, not followed by refining steps.

Herein, the ultra fine structure comprising ferrite grains is astructure mainly comprising ferrite grains. In this sense, the ultrafine structure comprising ferrite grains may be either single phasestructure of ferrite grains only, or may include a second phase ofcarbide, pearlite, martensite, or austenite. Further, fine carbonitridesand other precipitates may be included.

It is a third aspect of the invention to present a formed productcharacterized by having an ultra fine structure comprising elongatedferrite grains of average grain diameter of shorter diameter of 3 μm orless, it is a fourth aspect to present a formed product characterized byusing a steel having an ultra fine structure comprising elongatedferrite grains of average grain diameter of shorter diameter of 3 μm orless as raw material, and being produced by a forming step only, notfollowed by refining steps, it is a fifth aspect to present a formedproduct in the composition by wt. % of

C: 0.001% or more, 1.2% or less,

Si: 2% or less,

Mn: 3% or less,

P: 0.2% or less,

S: 0.1% or less,

Al: 0.3% or less,

N: 0.02% or less,

and a balance of Fe and inevitable impurities, and it is a sixth aspectto present a formed product of any one of the above products havingVickers hardness of 200 or more.

It is a seventh aspect of the invention to present a production methodfor a formed product without refining treatments comprising using asteel having an ultra fine structure comprising ferrite grains ofaverage grain diameter of 3 μm or less as raw material, and formingonly, not followed by refining, it is an eighth aspect to present aproduction method for a formed product in which using a steel having anultra fine structure comprising ferrite grains of average grain diameterof 1 μm or less as raw material, and it is a ninth aspect to present aproduction method for a formed product without refining treatmentscomprising using a steel having an ultra fine structure comprisingelongated ferrite grains of shorter grain diameter of 3 μm or less asraw material, by warm working or cold working of a material having ultrafine structure, and forming only, not followed by refining.

It is a tenth aspect of the invention to present a screw or boltcharacterized by having an ultra fine structure comprising ferritegrains of average grain diameter of 1 μm or less, it is an eleventhaspect to present a screw or bolt characterized by using a steel havingan ultra fine structure comprising ferrite grains of average graindiameter of 1 μm or less as raw material, and being produced by aforming step only, not followed by refining steps, it is a twelfthaspect to present a screw or bolt of high strength characterized byhaving a strength of 8.8 or more in JIS strength classification, it is athirteenth aspect to present a production method for a screw or boltwithout refining treatments comprising using a steel having an ultrafine structure comprising ferrite grains of average grain diameter of 1μm or less as raw material, and forming only by at least one process ofcold working and warm working, not followed by refining steps, and it isa fourteenth aspect to present a production method for a screw or boltin which using a steel having an ultra fine structure comprising ferritegrains of average grain diameter of 0.7 μm or less as raw material.

It is a fifteenth aspect to present a screw or bolt characterized byhaving an ultra fine structure comprising elongated ferrite grains ofaverage grain diameter of shorter diameter of 1 μm or less, it is asixteenth aspect to present a screw or bolt characterized by using asteel having an ultra fine structure comprising elongated ferrite grainsof average grain diameter of shorter diameter of 1 μm or less as rawmaterial, and being produced by a forming step only, not followed byrefining steps, and it is a seventeenth aspect to present a productionmethod for a screw or bolt, characterized by using a steel having anultra fine structure comprising elongated ferrite grains of graindiameter shorter diameter of 3 μm or less as raw material, by warmworking or cold working of material having ultra fine structure, andbeing produced by a forming step only, not followed by refining steps.

The present inventors have been studying intensively for years aboutminiaturization of crystal grains of ferrite steel. Miniaturization ofcrystal grain is a method of raising the strength of steel material onlyby miniaturization of crystal grains, without adding alloying elements,and it is the only method capable of enhancing the toughness at the sametime. It has been hence known to be the most ideal strength increasingmethod in steel materials.

Recently, inventors of the invention have accomplished to make thecrystal grains as ultra fine as 0.5 mm, which far exceeds the limit ofthe conventional fine size about 5 mm (Japanese Patent ApplicationLaid-Open (JP-A) No. 11-315342, (JP-A) No. 2000-309850, and (JP-A) No.2002-54670). By applying the ultra fine structure technology of crystalgrains in the material of high strength pressed product, it is expectedthat an enough enhancement of strength can be realized, and therebyreaching the present invention.

This time, as a result of additional intensive studies, if the ferritegrains are elongated in one direction, it has been found that pressedproducts, parts and members of various kinds can be obtained as highstrength material and high strength formed product, by controlling theshorter diameter. This discovery is very beneficial for productiontechnology.

In the case of machine screws of which screw shaft diameter is 2.0 mm orless, heat treatment such as hardening may be difficult due to theresidual stress after hardening or relation between hardening depth andscrew diameter or screw thread size in case of carburizing andquenching. The method of the invention is very effective when desired toobtain a member of high strength in spite of such small size.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the relation of ferrite grain diameter andtensile strength.

FIG. 2 is a photograph showing the appearance of steel bar of ultra finestructure of average grain diameter of 1 μm or less and SEM image.

FIG. 3 is a photograph showing the top view and side view of pressedproduct produced in an embodiment.

FIG. 4 (a) is a photograph of pressed product of the invention, and (b)is a photograph of sectional structural view of pressed part of aconventional pressed product.

FIG. 5 is a photograph of appearance of an example of embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention has the features as described above, and its preferredembodiments are described below.

The formed product of high strength presented by the invention isessentially made of a steel material having an ultra fine structurecomprising ferrite grains having an average grain diameter of 3 μm orless. It is also characterized by having an ultra fine structure offerrite grains of average shorter diameter of 3 μm or less. It has notbeen assumed at all for the formed product to have such ultra finestructure of ferrite grains, and it is realized for the first time inthe present invention.

Steel having an ultra fine structure comprising ferrite grains having anaverage grain diameter of 3 μm or less as raw material is notparticularly specified in production method or composition. The rawmaterial may be cold worked or warm worked, and ferrite grains may beelongated.

Preferably, the material may be bar or wire material made from thicksteel plate having an ultra fine structure proposed by the presentinventors (JP-A No. 2002-54670). That is, by introducing a strain largerthan a certain critical strain by applying multi-directional andmulti-pass pressing in warm working temperature region to thick steelplate, a steel material having a supercritical structure of averagegrain diameter of 1 μm or less can be used. In the steel having suchultra fine structure, for example, high strength is realized bypulverization of crystal grains, without making use of strengthenhancing mechanism by phase transformation. Accordingly, the pressedproduct made of such steel material can be produced without any refiningsteps of carburizing, hardening, or tempering, and can be presented as aformed product of high strength.

The high strength of the formed product of high strength in theinvention may be defined as Vickers hardness of 200 or more at ferritegrain average diameter of 3 μm or less.

In the aspect of composition, since strength enhancing mechanism byphase transformation is not utilized at all, addition of alloyingelements for enhancing the strength is not needed, and the steelcomposition is not limited at all, and steel materials of wide range ofcomposition may be used, such as ferrite single phase steel, austenitesingle phase steel, and other types of steel free from phasetransformation. A specific example is a composition by wt. % of

C: 0.001% or more, 1.2% or less,

Si: 2% or less,

Mn: 3% or less,

P: 0.2% or less,

S: 0.1% or less,

Al: 0.3% or less,

N: 0.02% or less,

Cr, Mo, Cu, Ni: 5% or less in total,

Nb, Ti, V: 0.5% or less in total,

B: 0.01% or less,

and a balance of Fe and inevitable impurities. These alloying elementsCr, Mo, Cu, Ni, Nb, Ti, V, B may be contained more than the specifiedrange as required, or may not be contained at all.

A screw of high strength of the invention is described below, and, forexample, a high strength screw mainly comprising 0.15% C-0.3Si-1.5% Mnmay be realized by tensile strength of 700 MPa at ferrite average graindiameter of 1.0μ, or 800 MPa at 0.7 μm as shown in FIG. 1. As highstrength screw sufficiently satisfying the strength of 8.8 or more inJIS strength classification, the average grain diameter of 0.7 μm orless may be presented. These values are only examples, and screws ofhigher strength can be presented in screws of different composition.

In the invention, the average grain diameter of ferrite grains isdefined by the cutting method in ferrite crystal grain testing method inJIS G0552, and the shorter diameter is the grain diameter of verticalsection of an elongated grain.

The production method for high strength formed product of the inventionis characterized by the process of forming step only such as pressing,without being accompanied by refining steps, by using a steel materialhaving an ultra fine structure of ferrite grains having an averageshorter diameter of 3 μm or less, in particular, ferrite grains having ashorter diameter of 3 μm or less.

The forming means is not particularly specified, and any known methodmay be employed depending on the desired standard and shape, such aspressing, forging, cutting, or header forming or thread rolling in thecase of screws. More specifically, using a bar or wire material of asteel having an ultra fine structure, the leading end of the material isprocessed to form a head of pressed piece, cut to a specified length,and pressed to form a pressed screw part.

By using a steel having an ultra fine structure as the material, thepresent inventors discovered that a pressed product of at least Vickershardness of 200 or more, or further JIS strength classification of 8.8or more (at least Vickers hardness of 250 or more) can be easilyproduced without requiring refining steps. That is, without requiringrefining steps such as carburizing, hardening or tempering, formedproduct, pressed product, part or member of high strength having highcore strength, tensile stress, and shearing stress can be produced.

The invention is more specifically described below while referring toexemplary embodiments.

EMBODIMENTS Embodiments 1 to 4

By melting a steel material having chemical composition as shown inTable 1, and introducing a strain larger than a critical strain in warmworking temperature region, a steel bar having an ultra fine structureof average grain diameter of 1 μm or less was prepared. The appearanceimage of the steel material and its scanning electron microscope (SEM)image are shown in FIG. 2. TABLE 1 [mass %] C Si Mn P S s.Al N 1 0.050.3 1.0 0.01 0.001 0.031 0.002 2 0.10 0.3 1.0 0.01 0.001 0.031 0.002 30.15 0.3 1.0 0.01 0.001 0.031 0.002 4 0.30 1.0 1.0 0.01 0.001 0.0310.002 5 0.45 0.3 1.0 0.01 0.001 0.031 0.002 6 0.76 0.3 1.0 0.01 0.0010.031 0.002 7 0.16 0.1 0.8 0.01 0.001 0.031 0.002 (SWCH16A)

This steel having an ultra fine structure was formed into a wirematerial of φ1.3 mm in diameter, and the leading end was formed to forma screw head, cut to a specified length, and rolled to form a screwhead, and a cross recessed pan head machine screw of M1.6 was produced(embodiments 1 to 4).

Top view and side view of obtained screw are shown in FIG. 3. By way ofcomparison, using chemical compositions 3 and 7 in Table 1, wirematerials of ferrite grain diameter of 20 μm were used, and screws wereproduced similarly (comparative examples 1, 2). Using chemicalcomposition 7 in Table 1, the material was formed, and refined byconventional method by hardening and tempering, and a screw wasproduced.

In these screws, the ferrite grain diameter of structure, tensilestrength, and screw core strength were measured, and results are shownin Table 2. In the screws of embodiment 1 and comparative example 1,screw sectional images are shown in FIG. 4 (a), (b), respectively. TABLE2 Ferrite Shape grain of di- Tensile Screw Compo- formed Heat ameterstrength core sition product treatment (μm) (MPa) strength Embodiment 3Screw None 0.7 807 269 1 Embodiment 2 Screw None 0.5 843 281 2Embodiment 3 Screw None 1.0 700 233 3 Embodiment 1 Screw None 0.5 800266 4 Comparative 3 Screw None 20 546 182 example 1 Comparative 7 ScrewNone 20 492 164 example 2 Comparative 7 Screw Quenching Marten- 730 242example 3 and site tempering

Screws of comparative examples 1, 2 not refined by hardening andtempering did not reach the Vickers hardness of 190, while the screws ofembodiments 1, 2, 4 exceeded the Vickers hardness of 250, and evenembodiment 3 had a high hardness exceeded 230. This hardness is same orhigher than that of the conventional refined screw of comparativeexample 3.

As known from FIG. 4, the high strength screw of embodiment 1 of theinvention has a very fine structure as compared with the screw ofcomparative example 1. In the high strength screw of embodiment 1, itwas completely free from martensite structure possibly caused byhardening.

Hence, the screws of the invention are proved to have a high strengthbecause of the ultra fine structure, without requiring refining steps.

The invention is not limited to these embodiments alone, but may bechanged and modified in various forms.

Embodiments 5 to 9

Using a wire material of φ8 mm in diameter of a steel having an ultrafine structure in the composition in Table 1, the leading end was formedto form a bolt head, cut to a specified length, and rolled to form M8bolts (embodiments 5, 7, 8). Using a wire material of φ3 mm in diameterof a steel having an ultra fine structure, the leading end was formed toform a head, cut to a specified length, and produced into rivets(embodiments 6, 9). FIG. 5 is a photograph showing the appearance ofembodiment 6.

By way of comparison, using chemical compositions shown in Table 1, wirematerials of ferrite grain diameter of 20 μm were used, and bolts andrivets were produced similarly (comparative examples 4 to 6). Usingchemical composition 7 in Table 1, the material was cold formed, andrefined by conventional method by hardening and tempering.

In these pressed products, the ferrite grain diameter of structure,tensile strength, and core hardness were measured, and results are shownin Table 3. TABLE 3 Ferrite Shape grain of di- Tensile Vickers Compo-formed Heat ameter strength hard- sition product treatment (μm) (MPa)ness Embodiment 3 Bolt None 0.6 810 275 5 Embodiment 3 Rivet None 0.5285 6 Embodiment 4 Bolt None 2.5 600 205 7 Embodiment 1 Bolt None 1 700235 8 Embodiment 6 Rivet None 0.7 340 9 Comparative 7 Bolt None 20 546182 example 4 Comparative 3 Rivet None 20 164 example 5 Comparative 3Bolt Quenching Marten- 730 242 example 6 and site tempering

Cold pressed bolts of comparative examples 4, 5 not refined by hardeningand tempering did not reach the Vickers hardness of 190, while theformed products of embodiments 5 to 9 exceeded the Vickers hardness of200, and in particular embodiments 5, 6, 9 exceeded 250. This hardnessis same or higher than that of the conventional refined pressed productof comparative example 6.

In the high strength formed products of embodiments 5 to 9, it wascompletely free from martensite structure possibly caused by hardening.

Hence, the pressed products of the invention are proved to have a highstrength because of the ultra fine structure, without requiring refiningsteps.

The invention is not limited to these embodiments alone, but may bechanged and modified in various forms.

INDUSTRIAL APPLICABILITY

As described specifically herein, the invention presents a high strengthformed product having high hardness and high toughness by the ultra finestructure, without resort to addition of alloying elements or refiningsteps, and also a production method for high strength formed productcapable of producting the same easily.

1. A formed product having an ultra fine structure comprising ferritegrains of average grain diameter of 3 μm or less.
 2. A formed productaccording to claim 1 comprising a steel having an ultra fine structurecomprising ferrite grains of average grain diameter of 3 μm or less asraw material, and being produced by a forming step only, not followed byrefining steps.
 3. A formed product comprising an ultra fine structurecomprising elongated ferrite grains of average grain diameter of shorterdiameter of 3 μm or less.
 4. A formed product according to claim 3comprising a steel having an ultra fine structure comprising elongatedferrite grains of average grain diameter of shorter diameter of 3 μm orless as raw material, and being produced by a forming step only, notfollowed by refining steps.
 5. The formed product as in claims 1 to 4,wherein the composition is, by wt. %, of C: 0.001% or more, 1.2% orless, Si: 2% or less, Mn: 3% or less, P: 0.2% or less, S: 0.1% or less,Al: 0.3% or less, N: 0.02% or less, and a balance of Fe and inevitableimpurities.
 6. The formed product as in claims 1 to 4, wherein theVickers hardness is 200 or more.
 7. A production method for a formedproduct without refining treatments comprising using a steel having anultra fine structure comprising ferrite grains of average grain diameterof 3 μm or less as raw material, and forming only, not followed byrefining.
 8. The production method for a formed product of claim 7,which comprises employing a steel having an ultra fine structurecomprising ferrite grains of average grain diameter of 1 μm or less asraw material.
 9. A production method for a formed product withoutrefining treatments comprising employing a steel having an ultra finestructure comprising elongated ferrite grains of shorter grain diameterof 3 μm or less as raw material, by warm working or cold working of amaterial having ultra fine structure, and forming only, not followed byrefining.
 10. A screw or bolt having an ultra fine structure comprisingferrite grains of average grain diameter of 1 μm or less.
 11. A screw orbolt according to claim 10 comprised of a steel having an ultra finestructure comprising ferrite grains of average grain diameter of 1 μm orless as raw material, and being produced by a forming step only, notfollowed by refining steps.
 12. The screw or bolt of high strength ofclaim 10 or 11, having a strength of 8.8 or more in JIS strengthclassification.
 13. A production method for a screw or bolt withoutrefining treatments comprising employing a steel having an ultra finestructure comprising ferrite grains of average grain diameter of 1 μm orless as raw material, and forming only by at least one process of coldworking and worm working, not followed by refining steps.
 14. Theproduction method for a screw or bolt of claim 13, in which using asteel having an ultra fine structure comprising ferrite grains ofaverage grain diameter of 0.7 μm or less as raw material.
 15. A screw orbolt having an ultra fine structure comprising elongated ferrite grainsof average grain diameter of shorter diameter of 1 μm or less.
 16. Ascrew or bolt according to claim 15 comprising a steel having an ultrafine structure comprising elongated ferrite grains of average graindiameter of shorter diameter of 1 μm or less as raw material, and beingproduced by a forming step only, not followed by refining steps.
 17. Aproduction method for a screw or bolt, which comprises employing a steelhaving an ultra fine structure comprising elongated ferrite grains ofgrain diameter shorter diameter of 3 μm or less as raw material, by warmworking or cold working of material having ultra fine structure, andbeing produced by a forming step only, not followed by refining steps.